The present invention relates to an exhaust gas purification system that purifies particulate matters from the exhaust gas discharged by diesel and other internal combustion engines using a diesel particulate filter and also to a control method thereof.
In the same way as for NOx, CO, and also HC etc., restrictions on the volume of particulate matters (hereinafter “PM”) discharged from diesel internal combustion engines grow severe every year. Techniques for collecting this PM in a filter known as a diesel particulate filter (hereinafter “DPF”) and for reducing the quantity thereof by discharging externally have been developed. There is a continuous regeneration-type DPF device using DPF devices and a catalyst as one of them.
However, even in continuous regeneration DPF devices, increasing exhaust gas pressure as a result of filter clogging has become a problem. Although the PM collected by the DPF is continuously burned and purified, and the DPF will self-regenerate while the exhaust gas temperature is approximately 350° C. or greater, when the exhaust gas temperature is low or the traveling condition of an internal combustion engine has a low NO discharge—for example, when an internal combustion engine has a continuously low exhaust gas temperature accompanied by idling or low load/low speed traveling, etc.—the temperature of the exhaust gas drops and the catalyst becomes inactive as a result of the low temperature thereof. Accordingly, as the oxidization reaction does not progress and NO becomes insufficient, the above-described reaction does not take place, the PM is not oxidized, and the filter does not regenerate. Accordingly, the accumulation of PM in the filter continues and clogging thereof is accelerated.
Countermeasures for clogging of the filter, an exhaust gas purification system of an engine as below has been proposed, for example, as disclosed in Japanese patent application Kokai publication No. 2003-83035. When the amount of clogging of the filter has exceeded a predetermined amount, exhaust gas temperature is forcibly raised and the collected PM then forcibly removed through being burned. As for means for detecting the filter clogging, there are some methods such as a method for detecting by the differential pressure before and behind the filter, and a method for detecting through determination of the PM accumulation quantity by calculating in accordance of a map data in which the PM quantity to be collected is previously set based on the engine operation state. Besides, based on the differential pressure detected by engine speed and by a pressure sensor, a clogging state is estimated.
Moreover, as an exhaust gas temperature raising means, several methods are considered such as a method using an intra-cylinder fuel injection control such as a multi injection (multistage delay injection) and a post injection in an intra-cylinder injection, a method using a fuel control for a direct fuel injection into an exhaust pipe, and a method for an acceleration of self-regeneration by closing an exhaust throttle valve installed on the downstream side of a DPF during the regeneration and keeping the temperature of the DPF constant.
This intra-cylinder injection control raises the temperature of exhaust gas by performing multi injection (multistage retarded injection) when the exhaust gas temperature is lower than the active temperature of an oxidation catalyst installed on the upstream side of a filter or an oxidation catalyst supported on the filter. Then, when the exhaust gas temperature rises higher than the active temperature, a post injection is performed. The fuel added into the exhaust gas by the post injection is burned at the oxidation catalyst to raise a temperature of the exhaust gas not less than the temperature that PM collected in a filter is burned. Thereby, PM is burned and removed to regenerate the filter.
Normally, in the case of these continuous regeneration-type DPF devices, when the accumulated PM quantity reaches a preset accumulated PM limit value, the operation condition of an internal combustion engine is automatically changed to a forced regeneration mode operation. In the case of the forced regeneration mode operation, the PM collected by the filter is oxidized and removed to perform regeneration treatment.
Moreover, in order to solve the problem of oil dilution due to a post injection for raising an exhaust gas temperature at the time of forced regeneration, the following method is considered. This method uses not only the automatic forced regeneration during vehicle travels but also a manual regeneration for performing forced regeneration in a vehicle stationary idling condition by warning a driver of a filter clogging by flashing a DPF lamp to stop the vehicle and turn on a manual regeneration switch.
Moreover, in the case of the prior art, the forced regeneration is interrupted when an engine key is turned off or a PTO (Power Take Off) is actuated. Moreover, when the engine key is turned on or the PTO is not actuated, the forced regeneration is restarted to perform the forced regeneration for the remained period of time of the previous actuation. Furthermore, in the case where the driver starts the vehicle while performing the forced regeneration by a manual regeneration in a vehicle stationary condition, and the forced regeneration is interrupted and the forced regeneration is performed for a remained period of time in the next regeneration.
However, when the forced regeneration is interrupted at the time of PTO actuation, there is a problem that PM is accumulated while it is interrupted and the collected PM cannot be sufficiently burned or removed by performing forced regeneration only for the remained period of time when restarting the next forced regeneration.
Similarly, when a vehicle starts to travel during the manual regeneration, the forced regeneration is interrupted. However, in the case where long distance is traveled by the next regeneration, PM is collected beyond expectation to cause a problem that the PM accumulated in a DPF cannot be sufficiently burned or removed by performing forced regeneration only for the remained period of time when restarting the next forced regeneration.
The following exhaust particulate purification system is proposed in Japanese patent application Kokai publication No. 1993-187221. In this system, in order to avoid a vehicle from being stopped during an automatic traveling regeneration, a driver operates a cancel button and the like to cancel the filter regeneration and to perform regeneration when the vehicle travels next time. Thereby, it is possible to shift the timing of the filter regeneration from the timing the vehicle stops.
However, this exhaust particulate purification system has the following problem. It is necessary for a driver to operate the cancel button by recognizing a lamp turned on and the like. Therefore, the driver feels troublesome and moreover. And the driver does not necessarily operate the cancel button. Therefore, it is impossible to securely avoid a vehicle from being stopped during an automatic traveling regeneration.